The present invention relates in general to the field of transmission line analysis and more particularly to a method and localization unit for locating one or more load coils within a transmission line.
Many copper-access network operators still have a considerable amount of customers subscribing only for telephony, i.e., plain old telephone service (POTS). Consequently, there is a great interest in estimating the suitability of twisted-pair copper lines for broadband services, provided by state-of-the-art digital subscriber line (DSL) technologies such as ADSL2+ and VDSL2. Common impairments that hinder or reduce the full potential of broadband access over these lines are, for example, crosstalk ingress due to line impedance imbalance, bridged-taps, and load coils. By far, the most severe impairment of these is the load coils, which are found on numerous lines in some countries.
A load coil is an inductive device that works like an impedance-matching transformer. Telecom operators used to install load coils in order to provide telephony to customers located far from the Central Office (CO). However, while the load coil reduces line attenuation at voice frequencies, it also drastically increases the attenuation at higher frequencies utilized by broadband services. Thus, the load coils must be removed before the line can be deployed for broadband services using the higher frequencies.
The operator has few options to pre-qualify the lines before investing in, and installing, broadband equipment. Essentially the options involve sending a technician into the field to conduct (expensive) manual testing, or to estimate the transmission line capacity from, for example, a database. Estimation is often not possible because in many cases the original lines were installed decades ago, and various kinds of modifications have been made throughout the years. Thus, databases of the access lines are often non-existent or inaccurate. Hence, a more attractive option for the operator is to employ automated one-port measurements from the CO, referred to as single-ended line testing (SELT).
The existing POTS transceivers have been designed to monitor and diagnose the narrow voice-band (0-4 kHz) by measuring elementary parameters such as DC/AC voltage and the resistance/capacitance at frequency zero between the wires of the line and between each wire and ground. These types of traditional measurements normally require metallic access to the line, which is commonly provided by expensive bypass-relays. Still, a large number of POTS transceivers are unable to detect and locate load coils. Moreover, conventional DSL modems, with a built-in SELT function, are not designed to detect the load coils since the access to the lower voice frequencies is normally blocked by the splitter filter, required by the co-existence of POTS and DSL transmission. Pre-qualification with DSL modems typically fails to distinguish the load coil from a cable-break or the far-end line termination. Consequently, there is a substantial benefit in performing these measurements via the already installed (narrow band) transceivers carrying POTS, i.e. via the telephone line cards.
There are several prior art methods oriented towards the detection of load coils. U.S. Pat. No. 4,620,068, U.S. Pat. No. 5,404,388, and U.S. Pat. No. 6,668,041 all describe methods that determine whether a line is equipped with load coil(s), i.e., whether a line is loaded or not, by detecting characteristics in the magnitude or the phase of the measured line. However, localization of the load coil(s) is not addressed.
In a co-assigned PCT Patent Application WO 2007/072191 filed on Dec. 20, 2006 and entitled, “Load Coil Detection and Localization”, both load coil detection and localization are considered. The contents of this application are incorporated by reference herein. However, in cases where the transmission line is not symmetric, the methods and devices disclosed in this application may fail to provide a sufficiently accurate load coil localization.
In the article entitled, “Automated Loaded Transmission-Line Testing Using Pattern Recognition Techniques” by William T. Bisignani, published in IEEE Transaction On Instrumentation and Measurement, IM-24, No. 1, 1975, automated testing of loaded lines is proposed. The method addresses load coil detection and localization by comparing the line under test with a pre-defined set of classes that represent possible line configurations. A decision space consisting of 20 regions (clusters) are utilized, each corresponding to one class. The number of pre-defined classes is kept low by assuming small deviation from the original load coil deployment rules. However, this assumption is not always valid due to changes of the access network by reconfigurations, displacement of the Cos, introduction of transceiver-cabinets closer to the customer, and the like. Thus the topology of the loaded transmission line may be more irregular than dictated by the original load coil deployment rules. Thus, this methodology would require an impractically large number of line-classes to accurately predict the location of the load coils.